The present invention is related to improved cascode amplifiers. More specifically, the present invention relates to a dynamic focus system for a cathode ray tube (CRT) which utilizes an improved cascode amplifier.
Typically in dynamic focus systems for CRTs, a variable amplitude voltage signal is provided at a focus electrode of the CRT. The desired variation of the focus voltage magnitude is a function of the position of the cathode ray tube electron beam with respect to the CRT screen. It is known that typically the desired variation of the focus voltage is defined by a parabola signal comprising a low frequency parabola envelope varying at the vertical sweep rate with a high frequency effective carrier signal varying at the horizontal sweep rate. U.S. Pat. Nos. 4,214,188 and 4,230,972, both assigned to the assignee of the present invention, generally discuss prior dynamic focus amplifier systems and the above noted parabola waveforms. These patents discuss providing a composite vertical and horizontal parabola signal by various techniques.
Typically a substantial voltage gain for the composite parabola signal is required so that a large peak magnitude variation for the focus voltage is achieved as the electron beam scans the CRT screen. An efficient way of implementing such a large voltage gain would be the use of a cascode amplifier with the lower (first) stage configured in a common emitter configuration, and the upper (second) stage configured in a common base configuration. One prior system which proposes such a configuration provides an AC ground path at the base of the upper transistor stage to insure the grounding of this base at the high horizontal sweep frequency. However, when substantial voltage gain is implemented by the cascode configuration and when a very large B+ voltage is utilized (thus requiring transistors with breakdown voltages able to withstand substantial reverse bias across their electrode terminals), it has been noticed that an undesirable phase shift occurs in the cascode amplifier configuration, especially when high horizontal sweep frequencies, such as 64 KHz, are utilized. For example, with a 64 KHz carrier (horizontal sweep) frequency and a 15 volt peak-to-peak variation in the input parabola signal, which resulted in a 500 volt peak-to-peak output parabola signal applied to the CRT focus voltage electrode, a 2 microsecond delay was noted. This has a detrimental affect since it results in the defocusing of the CRT electrode beam at the corners of the CRT screen since the focus voltage peaks will now not occur at the precise time at which the electron beam will reach the CRT screen corners. Utilizing a transistor having an improved frequency response for the upper transistor stage while still providing the desired reverse breakdown voltage capability was not feasible since transistors with such a combination of characteristics at a reasonable price were not available.